The present invention relates to an optical fiber connector embedded with a Bragg grating, and more specifically, to an optical fiber connector embedded with a Bragg grating, in which an optical filer provided with the Bragg grating is used as an optical communication filter capable of selectively reflecting or transmitting light of a desired wavelength band and an optical sensor for finely measuring variation of pressure, stress and tension in a system for monitoring cracks, displacement, deformation and stability of a large-scale structure such as a bridge or a tunnel, and the optical fiber connector may show stable optical characteristics by compensating reflective wavelength characteristics of the Bragg grating shifted by the stress applied to the optical fiber grating from outside and changes in external environment such as increase of surrounding temperature.
An optical fiber Bragg grating is a device for inducing change in the refractive index of an optical fiber core to have a regular period in the longitudinal direction using a photo-induced refractive index modulation effect generated when ultraviolet rays are radiated into an optical fiber of a silica family containing impurities such as germanium (Ga) in the optical fiber core. It has a characteristic of selectively transmitting light of a specific wavelength band by reflecting light of a Bragg wavelength bandwidth satisfying Bragg reflection conditions and transmitting light of a wavelength which does not satisfy the Bragg reflection conditions.
In an example of manufacturing an optical fiber Bragg grating, an optical grating structure is formed inside the optical fiber core if two beams create a periodic pattern through an interference phenomenon. Alternatively, a predetermined area of an optical fiber is melted using a certain heat source, and an area of the optical fiber heated by the heat source is tensioned in order to change the effective refractive index that a mode propagating light through the optical fiber core experiences, and thus a Bragg grating having a regular period is formed in the tensioned area.
Such an optical fiber Bragg grating filter can be easily manufactured in commercial optical fibers, and optical connection is very easy since the size and transmission characteristics are the same as those of existing optical fibers. Therefore, the optical fiber Bragg grating filter is very widely used for optical communication filters and optical sensors.
It is general that a reflection or transmission bandwidth should be about 50% of a wavelength division multiplexing (WDM) channel space, and crosstalk should be at least 25 dB or higher to suppress interference with other channels if the Bragg grating is desired to be used as an add/drop filter (ADF), a WDM channel filter or the like in the WDM communication field, and it is general that the length of a Bragg grating filter satisfying the specification described above should be 1 Cm or longer.
Meanwhile, in a Bragg reflection filter for stabilizing the wavelength of a pump laser having a bandwidth of 980 nm, the maximum reflectivity should be less than 5% within the reflection bandwidth. In this case, the length of the Bragg grating filter is generally less than 5 mm.
In addition, a Bragg grating filter for measuring variation of pressure, stress and tension is generally required to have a reflectivity around 50% in order to measure shift in the center wavelength with respect to changes in external environment. The optical fiber Bragg grating filter is required to have various characteristics and sizes depending on application fields and installation positions as described above.
Bragg reflection characteristics of the optical fiber Bragg grating, i.e., optical characteristics, sensitively change depending on changes in surrounding environment of the optical fiber Bragg grating, specifically external environment such as temperature, humidity, vibration and the like. Particularly, the characteristics are greatly affected by changes in the stress applied to the Bragg grating, which is caused by the change in external environment such as temperature which is direct cause of deformation of the optical fiber and connector according to the characteristics of material.
It is since that, in the optical fiber, change of the refractive index is induced in the optical fiber core by the thermo-optic effect of an optical fiber material when external temperature is changed, and the Bragg grating period is also changed by the thermal expansion property of the optical fiber material itself, and thus the value of the Bragg reflective wavelength is shifted. In addition, when an external stress is applied to the Bragg grating or stress distribution is changed around the Bragg grating due to the change in external temperature, the refractive index of the optical fiber core is changed by the photoelastic effect, and the Bragg grating period is also changed by the stress, and thus the value of the Bragg reflective wavelength is shifted.
An example of the characteristic of the Bragg reflective wavelength shifted according to change in external temperature of the optical fiber Bragg grating and the stress applied to the Bragg grating due to the change in external temperature will be described with reference to FIGS. 1 and 2.
FIG. 1 is a view showing the structure of an optical fiber cable 1 formed with a general shape Bragg grating. As shown in the figure, the optical fiber cable 1 includes an optical fiber 11 formed with a Bragg grating 10, an optical fiber coating 12 formed of a polymer material for protecting the Bragg grating 10 and the optical fiber 11, and an optical fiber cladding 13 formed of a high molecular polymer material.
FIG. 2 is a view showing the internal structure of a conventional optical fiber connector according to an embodiment of the present invention, in which the optical fiber cable 1 formed with an optical fiber Bragg grating as shown in FIG. 1 is inserted into and fixed to a structure 2 having a ferrule 20 and a socket 21.
The ferrule 20 is formed with an inclined surface 200 which is gradually narrowed from the inlet toward inside in order to easily insert the optical fiber 11 from the side surface of one end, and an optical fiber insertion hole 201 penetrating the side surface of the other end along the center of the axial direction of the ferrule 20 is formed on the inclined surface 200.
The socket 21 is formed with an insertion hole 210 for inserting the optical fiber cable 1 and the ferrule 20 penetrating both ends of the socket, and an extended end unit 211 formed on the outer periphery of one end where the optical fiber coating 12 and the optical fiber cladding 13 are inserted.
In the structure described above, the optical fiber 11 formed with the Bragg grating 10 is inserted into the optical fiber insertion hole 201 through a space where the inclined surface 200 of the ferrule 20 is formed, and the Bragg grating 10 is placed inside the optical fiber insertion hole 201. The optical fiber cladding 13 and the ferrule 20 are inserted into the insertion hole 210 formed in the socket 21. While the optical fiber cladding 13 is placed at one end of the socket 21 and the ferrule 20 is placed at the other end where the extended end unit 211 is formed, the optical fiber cladding 13 is fixed to one end of the socket 21 by thermosetting resin 212. While being placed in the optical fiber insertion hole 201 of the ferrule 20, the Bragg grating 10 is fixed by thermosetting resin 202.
FIG. 3 shows shift rates of the Bragg reflective wavelength (Δλ/ΔT) measured when external temperature is changed from 0 to 60° C. in the structure of FIG. 2.
Generally, when stress is not applied from outside, the shift rate of an optical fiber Bragg grating according to thermo-optic effect is 10 μm/deg. However, the measured shift rates are in a range of about 20 to 30 μm/deg although there is a difference according to temperature, and this is two or three times larger than the shift rate induced by the thermo-optic effect.
Observing the reflection spectra of the Bragg grating measured at external environment temperatures of 25 and 30° C. as shown in FIG. 3A, if surrounding temperature changes by 25° C., the Bragg center wavelength shifts to a wavelength of about 600 μm. This is caused by the fact that, other than the thermo-optic effect, when temperature of external environment is changed, stress is applied to the optical fiber Bragg grating due to the difference in thermal expansion coefficients of the materials surrounding the Bragg grating, and the grating period is changed thereby, and thus optical characteristics are changed.
A variety of methods have been proposed to reduce changes in the optical characteristics affected by dependency of the optical fiber Bragg grating on temperature change of external environment and stress.
Referring to the structure applied by William W. Morey et al. (Incorporated Bragg filter temperature compensated optical waveguide device, U.S. Pat. No. 5,042,898), proposed are a principle and a structure for making the Bragg reflective wavelength be independent from the temperature change of external environment by offsetting changes in the refractive index caused by the thermo-optic effect of a silica optical fiber with a variation rate of the optical fiber grating period caused by difference in thermal expansion coefficients, using heterogeneous materials having different thermal expansion coefficients, and a temperature compensation connection port is proposed as an embodiment thereof.
Referring to a paper issued by G. W. Yoffe et al. in Applied Optics, vol. 34, Issue 30, pp. 6859-6861, 1995, a structure has been manufactured using aluminum and silica based on the principle proposed by Morey as a temperature compensation connection port, and it is announced as a result that the degree of Bragg wavelength shift is reduced as much as 0.07 nm when the environment temperature varies from −30 to 70° C.
In addition, the structure applied by R. L. Lachance et al. (Adjustable athermal package for optical fiber devices, U.S. Pat. No. 6,907,164) also has proposed a temperature compensation structure of an optical fiber Bragg grating based on the principle proposed by Morey and presented a test result showing that the degree of Bragg wavelength shift is 0.1 nm when the temperature varies from −40 to 80° C.
In the cases described above, both of the structures offset the thermo-optic effect of the optical fiber Bragg grating by structurally combining heterogeneous materials having different thermal expansion coefficients. The principles are the same, and the proposed structures are different only in implementation methods.
FIG. 4 shows the configuration of a temperature compensation structure for illustrating the principle of compensating the optical fiber Bragg wavelength proposed in the documents described above. Referring to the figure, a structure formed of a material having a small thermal expansion coefficient and a temperature compensation connection port 31 formed of a material having a large thermal expansion coefficient are fixed to each other using thermosetting resin 32 or other mechanical method. The Bragg grating 10 of the optical cable 11 is inserted through the inner hole, and the optical cable 11 is fixed to the structure 30 and the temperature compensation connection port 31 using thermosetting resin 33 and 34. The temperature compensation connection port 31 is formed with an optical fiber support unit 310 protruded so as to be freely expanded or contracted by the change in external temperature without being interfered by the structure 30.
Here, the structure 30 is the ferrule 20 of a zirconia material or a composite structure including the ferrule 20 and the socket 21 of a metallic material as shown in FIG. 2, and its thermal expansion coefficient is α1. The thermal expansion coefficient of the temperature compensation connection port 31 is α2, and the effective thermal expansion coefficient of the Bragg grating 10 area according to the change in temperature of external environment is α. In order to compensate the temperature of the Bragg grating, α2 should be larger than α1.
When L1 is the distance between the position where the structure 30 and the temperature compensation connection port 31 are fixed to each other by the thermosetting resin 32 and the position where the optical fiber 11 is fixed to the structure 30 by the thermosetting resin 33, and Lα is the length of the Bragg grating 101 that is to be temperature-compensated, the mathematical expression shown below expresses the relation of α values and L1 and L2 for compensating temperature of the Bragg grating 10.α=(α1×L1−α2×L2)/(L1−L2)˜−9×10−6[1/deg]  Mathematical Expression 1
A variety of structures for compensating temperature of the Bragg grating can be derived from the mathematical expression. For example, the principle of mathematical expression 1 is applied to the structures proposed by Yoffe and Lachance. In an embodiment, a cylindrical housing and a temperature compensation connection port are formed using a metallic material such as silica, aluminum or the like and other fixing resin materials, and a bidirectional optical fiber pigtail is included to input and output light.
In FIG. 4, a material of the temperature compensation connection port 31 and a length of the protruded part 310 can be determined using mathematical expression 1. Accordingly, a value of L2, which is the length of the protruded part 310 of the temperature compensation connection port 31 with respect to length L1 of the structure 30, can be calculated using mathematical expression 1 based on Table 1 which summarizes thermal expansion coefficient values of available materials.
Thermal expansion coefficients of various materials that can be used for the structure 30 are shown in Table 1.
TABLE 1MaterialCTE[10−6/deg]Aluminum (Al)23Brass19SUS30417Acetal (POM)100~150Polycarbonate (PC)60~70Polyimide55Zirconia10
For example, when the material of the structure 30 is zirconia and the material of the temperature compensation connection port 31 is aluminum and brass in FIG. 4, a result of calculating values of structural variables L2, L1 and Lα defined in FIG. 4 is summarized in Table 2 shown below.
TABLE 2Zirconia/BrassZirconia/AluminumL2 (mm)L1 (mm)Lα (mm)L2 (mm)L1 (mm)Lα (mm)0.50.840.340.50.740.2411.680.6811.470.471.52.5210.21.52.210.7123.360.3622.940.942.54.201.702.53.681.1835.042.0434.411.413.55.882.383.55.151.6846.722.7245.881.884.57.563.064.56.622.1258.403.4057.362.365.59.243.745.58.092.59610.084.0868.832.836.510.924.426.59.563.06711.764.76710.363.307.512.605.107.511.033.53813.455.45811.773.778.514.295.798.512.514.01915.136.13913.244.249.515.976.479.513.984.481016.816.811014.714.71
In Table 2, a range of L2 values is determined as 0 to 5 mm considering the length of the ferrule in an optical fiber connector of an LC or MU type. In the case of an optical fiber connector ferrule of an SC or FC type, the range of L2 values can be as wide as 0 to 10 mm.
Here, Lα denotes the actual length of the optical fiber Bragg grating and has a value of L1-L2.